A motor may be combined with a compressor, such as a centrifugal compressor, in a single housing to provide what is known as a motor-compressor device. Motor-compressors are often used in subsea environments in the production or transport of hydrocarbons. Via a shared rotating shaft supported by a rotor-bearing system, the motor drives/rotates the compressor in order to generate a flow of compressed process gas. As the motor drives the compressor, heat is generated by the electrical systems that are characteristic of electric motor drivers. Heat is also generated through the windage friction resulting from the rotating components operating in pressurized gas. If this heat is not properly managed, it adversely affects the performance of the motor and can damage the insulation of the stator. Increased temperatures can also adversely affect the rotor-bearing systems of both the compressor and motor, thus leading to bearing damage and/or failure.
One common method of cooling the motor and bearings in a subsea motor-compressor is to use an open-loop cooling circuit where gas is drawn from the process stream at some point in the compression process. This gas is then passed through the motor and bearing areas to absorb heat, and subsequently directed back to the suction inlet of the compressor. The benefits of these systems include low part count and some protection against hydrate formation throughout the cooling circuit. These systems typically do not have active temperature control for the cooling loop due to reliability concerns, and therefore must float the temperature of the cooling loop on the heat input from the motor and the heat rejection to the cooling gas. The lower limit on cooling loop temperature for open-loop systems is simply the process gas side stream temperature. The cooling loop cannot go below this temperature, which is normally above hydrate formation values.
Unfortunately, along with these benefits there are two significant problems with the open-loop cooling circuit approach to subsea motor-compressor cooling. The first is long term contamination of the motor and bearing areas. To limit the amount of debris circulated through the cooling loop, some form of filter or separation device must be used. However, even with very good filtration of the gas, a certain amount of debris inevitably passes into the cooling circuit. This debris tends to drop out at low velocity areas of the cooling circuit, and over time, with continual accumulation of debris, significant degradation in cooling effectiveness occurs. The second problem is with the filter/conditioning device, which not only adds complexity, size, and cost to the subsea module, but also has material build-up and plugging issues as well. The filter/conditioning device either has to have lower separation performance, such as with a cyclonic separator, which leads to quicker fouling of the motor and bearings, or it must be a high-performance filtration device, which requires periodic cleaning, flushing, or other servicing, each of which are expensive and problematic in a subsea setting.
Another method of cooling the motor and bearings in a subsea motor-compressor is to use a semi-closed loop cooling circuit. In the semi-closed loop cooling systems, the cooling circuit is generally isolated by a shaft seal that limits fluid communication with the process stream, and thus has a smaller chance of accumulating contaminating material. Only a small amount of process gas (typically less than 1% of the main gas flow) is fed into the cooling circuit from the process stream to make-up for seal leakage. The semi-closed loop system often uses a small blower to circulate the cooling gas through the cooling circuit. In subsea applications, the cooling gas is typically cooled in a sea water-cooled heat exchanger.
Although the semi-closed loop cooling system minimizes debris build-up in the cooling circuit, it has its own shortcomings as well. Using a sea water-cooled heat exchanger to cool the cooling gas introduces a significant risk of hydrate formation in the cooling circuit. This is especially true for a floating temperature control scheme tied to the sea water-cooled heat exchanger and while running at low load conditions. As a result, the cooling gas can actually approach the temperature of the sea water because of its thermal contact in the heat exchanger.
There is a need, therefore, for an improved, compact, and more robust cooling system for a subsea motor-compressor arrangement that will not be susceptible to the drawbacks of the prior systems described above.